For example, a plunging type constant velocity universal joint that is axially displaceable while forming an operating angle but forms a relatively small maximum operating angle is generally assembled on an inboard side (differential side) of an automotive front drive shaft. Further, a fixed type constant velocity universal joint that can form a large operating angle but is not axially displaceable is generally assembled on an outboard side (wheel side) of the automotive front drive shaft because the wheel is steered on the outboard side.
An example of the fixed type constant velocity universal joint to be used on the outboard side is described with reference to FIG. 19A, which is a vertical sectional view of a state in which a constant velocity universal joint 101 forms an operating angle of 0°, and to FIG. 19B, which is a schematic view of a state in which the constant velocity universal joint 101 forms the maximum operating angle. The constant velocity universal joint 101 is a Rzeppa type constant velocity universal joint of an eight ball type, and mainly includes an outer joint member 102, an inner joint member 103, balls 104, and a cage 105. Eight track grooves 107 are formed equiangularly in a spherical inner peripheral surface 106 of the outer joint member 102 so as to extend along an axial direction. Track grooves 109 opposed to the track grooves 107 of the outer joint member 102 are formed equiangularly in a spherical outer peripheral surface 108 of the inner joint member 103 so as to extend along the axial direction. Each ball 104 is arranged between the paired track grooves 107 and 109 (opposed to each other in a radial direction) of the outer joint member 102 and the inner joint member 103. The cage 105 for holding the balls 104 is arranged between the spherical inner peripheral surface 106 of the outer joint member 102 and the spherical outer peripheral surface 108 of the inner joint member 103.
As illustrated in FIG. 19A, the cage 105 has a spherical outer peripheral surface 112 fitted to the spherical inner peripheral surface 106 of the outer joint member 102, and a spherical inner peripheral surface 113 fitted to the spherical outer peripheral surface 108 of the inner joint member 103. The spherical outer peripheral surface 112 and the spherical inner peripheral surface 113 each have a curvature center formed at a joint center O. On the other hand, a curvature center Oo of a ball raceway center line x of each track groove 107 of the outer joint member 102 and a curvature center Oi of a ball raceway center line y of each track groove 109 of the inner joint member 103 are offset to both sides in the axial direction by equal distances with respect to the joint center O. Therefore, when the joint forms an operating angle, the balls 104 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 102 and the inner joint member 103. As a result, rotational torque is transmitted at a constant velocity between the two axes.
As illustrated in FIG. 19B, a maximum operating angle θmax, which is defined as a main function of the fixed type constant velocity universal joint 101, depends on an angle causing interference between an inlet chamfer 110 formed at an opening rim (inner rim portion) of the outer joint member 102 and a shaft 111. In order to secure permissible torque to be transmitted, an axial diameter d of the shaft 111 is determined for each joint size. When a large inlet chamfer 110 is formed, the length of each track groove 107 of the outer joint member 102, on which the ball 104 is brought into contact (hereinafter referred to as “effective track length”), is insufficient. As a result, the ball 104 may drop off the track groove 107, and the rotational torque cannot be transmitted. Therefore, how the inlet chamfer 110 is formed while securing the effective track length of the outer joint member 102 is an important factor in securing the operating angle. In the constant velocity universal joint 101 illustrated in FIGS. 19A and 19B, the curvature center Oo of the ball raceway center line x of the track groove 107 is offset to an opening side. Thus, there is an advantage in terms of the maximum operating angle, and the maximum operating angle θmax is approximately 47°.
Further, as compared to a related-art constant velocity universal joint of a six ball type, the above-mentioned constant velocity universal joint of the eight ball type has a smaller track offset amount, a larger number of balls, and has a larger diameter. Thus, it is possible to attain a highly efficient constant velocity universal joint that is lightweight and compact, and is suppressed in torque loss. However, at an operating angle of 0°, wedge angles formed between the paired track grooves 107 and 109 of the outer joint member 102 and the inner joint member 103 are opened toward the opening side of the outer joint member 102. Therefore, due to axial forces applied from the track grooves 107 and 109 to the balls 104, loads to be applied to the spherical contact portions 106 and 112 of the outer joint member 102 and the cage 105 and the spherical contact portions 108 and 113 of the inner joint member 103 and the cage 105 are generated in a certain direction. Thus, this structure leads to restriction on achieving even higher efficiency and less heat generation.
Therefore, in order to achieve even higher efficiency and less heat generation, a fixed type constant velocity universal joint 121 of a track groove crossing type illustrated in FIGS. 20A and 20B has been proposed (see, for example, Patent Literature 1). FIG. 20A is a vertical sectional view of a state in which the constant velocity universal joint 121 forms an operating angle of 0°, and FIG. 20B is a schematic view of a state in which the constant velocity universal joint 121 forms a high operating angle. As illustrated in FIG. 20A, the constant velocity universal joint 121 mainly includes an outer joint member 122, an inner joint member 123, balls 124, and a cage 125. Although illustration is omitted, in the constant velocity universal joint 121, planes including ball raceway center lines x of eight track grooves 127 of the outer joint member 122 and a joint center O are inclined with respect to a joint axial line n-n with their inclination directions opposite to each other in the track grooves 127 adjacent to each other in a peripheral direction. In addition, each track groove 129 of the inner joint member 123 has a ball raceway center line y, which is formed so as to be mirror-image symmetrical with the ball raceway center line x of the paired track groove 127 of the outer joint member 122 with respect to a joint center plane P at the operating angle of 0° (plane including the joint center O at the operating angle of 0° and extending in a direction orthogonal to the joint axial line n-n).
As illustrated in FIG. 20A, each of the track groove 127 formed in a spherical inner peripheral surface 126 of the outer joint member 122 and the track groove 129 formed in a spherical outer peripheral surface 128 of the inner joint member 123 extends into an arc shape along the axial direction, and a curvature center of each of the track grooves 127 and 129 is positioned at the joint center O. Each ball 124 is interposed in a crossing portion between the paired track grooves 127 and 129 (opposed to each other in a radial direction) of the outer joint member 122 and the inner joint member 123. The cage 125 for holding the balls 124 is arranged between the spherical inner peripheral surface 126 of the outer joint member 122 and the spherical outer peripheral surface 128 of the inner joint member 123. The cage 125 has a spherical outer peripheral surface 132 fitted to the spherical inner peripheral surface 126 of the outer joint member 122, and a spherical inner peripheral surface 133 fitted to the spherical outer peripheral surface 128 of the inner joint member 123. The spherical outer peripheral surface 132 and the spherical inner peripheral surface 133 each have a curvature center formed at the joint center O. In the constant velocity universal joint 121, curvature centers of the ball raceway center lines x and y of the track grooves 127 and 129 of the outer joint member 122 and the inner joint member 123 are arranged at the joint center O. However, the paired track grooves 127 and 129 cross each other, and the balls 124 are interposed in those crossing portions. Therefore, when the joint forms an operating angle, the balls 124 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 122 and the inner joint member 123. As a result, rotational torque is transmitted at a constant velocity between the two axes.
In the above-mentioned fixed type constant velocity universal joint 121 of the track groove crossing type, the track grooves 127 of the outer joint member 122 that are adjacent to each other in the peripheral direction are inclined in the opposite directions. Further, the track grooves 129 of the inner joint member 123 that are adjacent to each other in the peripheral direction are inclined in the opposite directions. Therefore, forces in the opposite directions are applied from the balls 124 to pocket portions 125a of the cage 125 that are adjacent to each other in the peripheral direction. Due to the forces in the opposite directions, the cage 125 is stabilized at the position of the joint center O. Thus, a contact force between the spherical outer peripheral surface 132 of the cage 125 and the spherical inner peripheral surface 126 of the outer joint member 122, and a contact force between the spherical inner peripheral surface 133 of the cage 125 and the spherical outer peripheral surface 128 of the inner joint member 123 are suppressed. Accordingly, the joint is smoothly operated under high load and in high speed rotation. As a result, torque loss and heat generation are suppressed, and the durability is enhanced.